This invention relates generally to precision assembly of multiple optical fibers in optical devices, and more particularly, to fabrication of differential fiber optic sensors, such as, for example, those used for sensing the position of ink droplets during flight.
Ink jet printers of the continuous stream type employ printheads having multiple nozzles from which continuous streams of ink droplets are emitted and directed towards a recording medium. Printing information is transferred to the droplets of each stream by electrodes which charge the droplets passing thereby. This permits each droplet to be individually charged so that it may be positioned as a distinct location on the recording medium different from all other droplets or sent to the gutter. As the droplets proceed in flight from the charging electrodes towards the recording medium, they are passed through an electric field which deflects each individually charged droplet in accordance with its charge magnitude to specific pixel locations on the recording medium. Thus, to calibrate the ink jet printer, the ink droplet trajectories must be determined and adjusted. One such means of calibrating the ink droplets is described in U.S. Pat. No. 4,225,754 to Crean et al.
U.S. Pat. No. 4,255,754 to Crean et al discloses the use of paired photodetectors to sense ink droplets, one each for two output fibers that are used to generate an electrical zero crossing signal. The zero crossing signal is used to indicate alignment or misalignment of a droplet relative to the bisector of a distance between two output fibers. The sensor of this patent employs one input optical fiber and at least two output optical fibers. The free ends of the fibers are spaced a small distance from each other; the free end of the input fiber is on one side of the flight path of the droplets, and the free end of the output fibers are on the opposite side. The remote end of the input fiber is coupled to a light source, such as an infra-red light emitting diode. The remote ends of each output fiber are coupled to separate photodetectors such as, for example, a photodiode responsive to infra-red radiation. The ink is substantially a dye dissolved in water and is, of course, transparent to infra-red light, thus reducing the problems of contamination usually associated with ink droplet sensors. The photodiodes are coupled to differential amplifiers, so that the output of the amplifiers are measurements of location of the droplets relative to the bisector of the distance between the two output fiber ends confronting the associated input fibers and droplets passing therebetween. Amplifier outputs are used in servo loops to position subsequently generated droplets to the bisector location. This process enables droplets from each stream to be precisely positioned to multiple pixel positions within a segment of a print line that extends across the recording medium. Consequently, the print line segments of the adjacent droplet streams are said to be stitched.
The Crean et al optical fiber sensors involve a large number of fibers with each of the two output fibers being separated into groups for termination at first and second photodetectors. If the two output fibers for each sensor are identified as A and B fibers, all of the A fibers share the same photodetector, and all of the B fibers share the same second photodetector. Since the light collecting ends of the plural sets of A and B fibers lie in the same plane, the A fibers must cross over the B fibers for the two types to be grouped together. That is, the fibers are organized into groups that intersect each other thereby necessitating that the A fibers be crossed over the B fibers or vice versa. This can be done with individual fibers but makes for difficult assembly of a large number of sensors.
U.S. Pat. No. 4,344,078 to Robert D. Houston, discloses the offsetting of the light collecting ends of each A and B fiber into parallel and separate planes, at least at the sensing zone. In one embodiment, the A fibers of multiple fiber pairs are formed on a support surface of a single substrate member. The A fibers are coated over with an appropriate separation material creating a second laminated support surface. The B fibers are formed on this second support surface. In another embodiment, the A, B, and input fibers are formed on separate substrates. The A and B fibers are then oriented at the sensing zone. Detection circuits coupled to the remote ends of the A and B fibers store the signals associated with a droplet shadow striking the A and b fibers. The storage is provided because the two signals are generated at different times. The delay is due to the separation between the A and B fibers along the direction of droplet flight. Thus, the patent to Houston discloses one method for practicing the sensing technique disclosed in the Crean et al patent. Further, the light source and receiver of Houston sensor include optical paths which are photofabricated to a support substrate.
U.S. Pat. No. 4,410,895 to Robert D. Houston et al, discloses the use of an optical masking technique which allows utilization of bulk optical fibers having cross-sectional areas greater than the dimension required by the ink droplet sensing apparatus. These bulk fibers can be more easily routed away from the sensing sights and can be manually flexed and bent as needed to route them to the light intensity detecting circuitry. Masks are interposed between the input and output light fibers to define optical sending and receiving sights having areas less than the fiber areas which they mask. The masking of these fibers is accomplished by positioning electroformed metal masks over the end surface of these output fibers. The light transmitting regions as defined by these masks are separated along the dimension of the path travel all being closely spaced in the direction of droplet deflection. The closely adjacent positioning of the output fibers enhances sensitivity to aid in the stitching together of the ink droplets and a printing array. According to one assembly technique, the input and output fibers are manually mounted to a mounting plate through which mounting holes are drilled. The optical fibers are inserted through the plate and then secured in place by a plotting compound which firmly secures the fibers to the plate without limiting bending or flexing of the optical fibers.
Fiber optical devices requiring a plurality of precisionally aligned optical fibers, such as, for example, aligned sending and receiving fibers which respectively interface with a light source and light responsive electronic components are generally manually assembled. Current methods of achieving appropriate alignment require dexterous individuals to position tediously the optical fibers into fabricated channels or grooves one fiber at a time through the use of a microscope. As each fiber is placed in a channel, they are adhesively secured. Such manual assembly is not only slow and costly, but are subject to damage by the person fabricating the optical fiber array. One broken or misaligned optical fiber reduces the effectiveness of the array, especially if used as a drop sensor in an ink jet printer.
Therefore, a need remains for the development of an automated technique for the precision aassembly of a plurality of aligned optical fibers which provide minimum labor costs with substantially zero human error, while maintaining the highest quality and productivity.